Sinewave inverter using hybrid regulator

ABSTRACT

A sinewave inverter for converting unstable DC voltage from a variable source such as batteries, fuel cells, wind mills and the like into a distortionless sinusoidal AC voltage of constant amplitude and constant frequency is provided. This pure sinewave inverter with line and load regulated voltage is obtained by using a combination of a hyperbolic frequency modulator with a sinusoidal pulsewidth modulator in the inverter circuit.

FIELD OF THE INVENTION

The present invention relates to a sinewave inverter for converting DCto AC voltage, and more particularly to a kilowatts pure ordistortionless sinewave inverter using a hybrid regulator comprising ahyperbolic frequency modulator combined with a sinusoidal pulsewidthmodulator.

BACKGROUND OF THE INVENTION

DC to AC inverters appeared about 60 years ago, mainly for aerospaceapplications. They used various voltage mode or current mode switchingtechniques, such as saturating magnetic core topologies or two currentsources as disclosed, for instance, in U.S. Pat. No. 4,415,962.

Such inverters were simple in nature, but due to the non-linearphenomenon appearing in the magnetic core, they were difficult toregulate and predict. Filtering was not straightforward, because filtershad to work with widely varying input and output impedances.

With the advent of microprocessors, sampling theories with custom madesoftware algorithmns have been used to produce inverters withdistortionless and regulated sinewaves. This approach works fairly wellat low powers (below 300 watts), but becomes complicated and not tooreliable at higher powers, because of the response of inductive powerchokes and transformers to the sampling frequency, especially whenloading is varying by large increments. The net result of this is highdevelopment, production and maintenance costs (around $1 to $2/watt)which amounts to $5000 to $10,000 for a 5 kw inverter. This is notcommercially viable.

There is thus a need for a commercially viable pure sinewave inverterhaving essentially the following specifications:

1. Input unstable DC voltage (typically±50%) provided by batteries, fuelcells, wind mills, photovoltaic cells, solar cells, and the like;

2. Output: constant amplitude (typically 115 VAC±5%) and constantfrequency (typically 60 hz+0.5 hz);

3. Pure sinewave: with typically less than 2% harmonic distortion;

4. Efficiency: at least 98%; and

5. Cost: low cost (typically in the range of $0.05/regulated watt).

At present, the above conditions cannot be achieved simultaneously,particularly in so far as the low cost is concerned for the highefficiency and other features set out above.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a sinewaveinverter using a hybrid regulator for converting a direct current (DC)voltage to an alternating current (AC) voltage using a hyperbolicfrequency modulation, i.e. a 1/x frequency modulation combined with asinusoidal pulsewidth modulation to achieve the five inverter conditionsmentioned above.

In U.S. Pat. No. 5,357,418 and the corresponding Canadian Patent No.2,054,013 issued to the same inventor, which are incorporated herein byreference, it is already explained why, if a high frequency is made tovary inversely proportional (hyperbolic function) to the amplitude of arectified and filtered AC and is subsequently used to switch the FETs ofa push-pull device, the following desirable effects are produced:

after high speed rectification and filtering at the secondary of thetransformer, a constant DC voltage is produced irrespective of the linevoltage variations; and

the value of this DC voltage can be set merely by increasing ordecreasing the pulsewidth from 0 to pwmax, with “pwmax” being the periodof the variable frequency.

This is basically an open loop regulation scheme, the purpose of whichis to obtain line regulation only.

After line regulation is obtained, a FET type linear regulation stage isadded to take care of the load regulation. Due to the line-regulation,the drop across pass element is kept to a minimum and hence linearquality regulation is obtained for the fill load. At no load, the dropacross the pass element increases, but current is negligible and lossesin the pass element are also negligible.

Moreover, whatever the complexity of the load (inductive, capacitive,complex, abruptly varying, etc.), it does not interfere with the highfrequency feedback loop or the complex impedances of the pre-regulator,avoiding a severe problem that usually exists with conventionalswitching regulators.

Thus, linear quality regulation (line and load) with high efficiency ismade possible with this topology.

It has been surprisingly found that the converter topology describedabove, based on the use of 1/x or hyperbolic frequency modulation canalso produce a sinewave inverter topology that essentially complies withthe five above mentioned conditions, when it is combined with a sinepulsewidth modulation. In essence, the hybrid combination ofhyperbolically modulated frequency combined with sinusoidally modulatedpulsewidth produces a high efficiency linearly regulated AC supply fromany type of DC input.

Thus, the present invention provides for a sinewave invertercharacterized in that it comprises a combination of a hyperbolicfrequency modulator with a sinusoidal pulsewidth modulator adapted toproduce a line and load regulated distortionless sinusoidal voltage.

Preferably, the hyperbolic frequency modulator is adapted to producehigh frequency which is exactly inversely proportional to a variableinput DC voltage, and the pulsewidth modulator is adapted to produce apulsewidth exactly proportional to the voltage of a sinusoidaldistortionless reference voltage from a pure sinewave modulator.Moreover, the sinusoidal pulsewidth modulator may be adapted to producea voltage which is exactly proportional to the voltage from a grid,thereby enabling the inverter to produce AC voltage which exactly mimicsthe grid voltage amplitude, frequency and waveshape and hence candeliver power to the grid.

Furthermore, the inverter of the present invention may comprise aprecision full wave rectifier adapted to provide a reference signal froma master-slave arrangement suitable to deliver any desired power output.

In a preferred embodiment, the present invention provides a sinewaveinverter using a hybrid regulator for converting DC input voltage from avariable DC source to pure sinewave line and load regulated AC voltageat the output, which comprises:

(a) a hyperbolic frequency modulator for producing high frequency whichis exactly inversely proportional to the variable in put DC voltage;

(b) a voltage divider for feeding a faction of the input voltage to saidhyperbolic frequency modulator;

(c) a sinusoidal pulsewidth modulator producing a pulse triggered by themodulated frequency from the hyperbolic frequency modulator and whosewidth is exactly proportional to the reference half sinewave amplitudefrom an internal or external sine reference source and a precision fullwave rectifier;

(d) a pair of push-pull switching FETs connected to a bi-phase togglewhich is triggered by the sinusoidal pulsewidth modulator and thehyperbolic frequency modulator and providing a flip-flop for the twophases of FET drives of the push-pull stage;

(e) a high frequency transformer following the push-pull stage connectedto an integrating choke which itself is connected to a FET pass elementused to produce a low drop linear regulator which is provided with anamplifier whose reference input receives half-sine waves from the linearregulator; and

(f) a FET synchronous bridge for converting the amplified half sinewaves obtained from the linear regulator into full sinewaves of ACvoltage at the output of the inverter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the appendeddrawings, in which:

FIG. 1 is a block diagram representing a preferred embodiment of theinvention;

FIG. 2 is a graph showing main waveforms when the primary DC sourcedelivers its minimum DC voltage; and

FIG. 3 is a graph showing main waveforms when primary DC source deliversits maximum DC voltage.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, it shows a block diagram representing a preferredembodiment of the inverter according to the present invention. The input10 to the inverter is a variable DC source, such as a battery bank, fuelcell, solar cell bank and the like. DC voltage variations can beessentially limitless, but for the purposes of this embodiment, theminimum voltage is chosen to be 50 VDC and the maximum voltage 100 VDC.This unstable DC power is connected to the inverter at entry points “A”and “B”. Power out can also be any desired value, but herein it ischosen to be 115 VAC, 60 hz, 45 amps, i.e. 5 kilowatts. It is providedat exit points “E” and “F” of the inverter where the user's appliancesrequiring stable AC power are connected.

A voltage divider 11 is provided for feeding a fraction of the linevoltage from input 10 to a hyperbolic frequency modulator 20. Twopush-pull switching FETs or FET modules 12 are connected to a bi-phasetoggle 22 which is a flip-flop that produces phases A and B for the FETdrives of the push-pull stage. These phases are 60 hz square-pulsesoriginating from sync squarer 23 which are used to reconstruct thecomplete power sinewave (i.e. positive and negative alternances). Forthis embodiment 200V, 50 amps FETs have been chosen. Then push-pullstage is followed by a high frequency transformer 13 which, for thisembodiment has been chosen as a 5 kw, 100 kilohertz transformer. Therole of the transformer is to isolate the DC input from the AC output,and to raise the voltage levels to the correct 160 v peak necessary to a115 vrms power sinewave. This is followed by an integrating ferritechoke 14 which is used for averaging high frequency pulses in order toproduce the low frequency (60 hz) and which in this case is a 300microhenries choke, connected to a FET pass element 15 located betweenpoints “C” and “D” of the inverter and used to produce a low drop linearregulator. A standard op amp error amplifier 16 is provided for thelinear regulator, whose reference input receives, in this case, 60 hzhalf-sine waves at 10v amplitude. This is followed by a FET synchronousfull bridge 17 used to convert unidirectional half sinewaves into fullsinewaves, and leading to the user's load 18 which can be any compleximpedance. The AC power out at points “E” and “F” can also be fed to agrid 19 if the inverter is used to feed such a grid.

The hyperbolic frequency modulator 20 produces a frequency k/v where vis proportional to the line voltage and the hyperbola curve fit ispreferably exact within ±1%. The frequency modulated voltage from themodulator 20 is fed to a sinusoidal pulsewidth modulator 21 whichproduces a pulse triggered by the k/v frequency and whose width isproportional preferably within ±1% to the reference half sinewaveamplitude produced by a precision full wave rectifier 24 which is a lowpower (normally 100 milliwatt) rectifier with no offset and having astandard management with the op amp 16. It also provides a referencesignal for any master-slave arrangement that might be needed for powersexceeding 5 kilowatts. Thus, the hyperbolic frequency modulator 20triggers the sinusoidal pulsewidth modulator 21 to obtain a frequencythat varies hyperbolically and a pulsewidth that varies sinusoidally.The combination of these two functions produces regulation and sinewaveoutput. The hyperbolic frequency modulator 20 also sends synchronizingsignals to the bi-phase toggle 22 to produce bi-phase signals. The syncsquarer 23 is a simple pulse shaping circuit producing thesynchronization pulses for the FET synchronous full bridge 17 thatconverts unidirectional half sinewaves into full sinewaves.

As an internal sine reference to the precision full wave rectifier 24,there may be provided a pure sinewave modulator 25, which is a highpriority, high stability, low power (100 milliwatts, 60 hz) sinewavegenerator, such as Wien bridge or a crystall controlled sinewavegenerator.

Moreover, an external sine reference from the grid 16 may be provided,which is a small 1 watt 60 hz transformer that will output a low voltagesignal mimicking exactly the grid voltage. This signal is subsequentlyfed as a reference to the precision full wave rectifier 24 and tosinusoidal pulsewidth modulator 21 and the sync squarer 23, exactly asthe internal reference. The net effect is that the output of theinverter will also exactly mimic the voltage of the grid 16 even if thegrid voltage is not exactly sinusoidal. This feature is particularlyinteresting if the inverter has to deliver power to the grid. Theapproximate component cost of a 5 kw inverter having the arrangementdescribed above and illustrated in FIG. 1 is as follows: 7 power FETs at$4.00 each $28.00 1 5 kw transformer, 100 Khz $50.00 20 standard CMOSand linear Ics $10.00 2 fast rectifiers $8.00 1 small transformer, 1 va$5.00 1 choke, 300 microhenries $10.00

The total of $111.00 is very close to the $ 0.05/watt objectivementioned above. It should be noted that no software is implied in thisdesign and troubleshooting can be readily accomplished by any technicianhaving reasonable knowledge of analog circuits.

Referring to FIG. 2, it shows the waveforms occurring at differentpoints of the block diagram of FIG. 1 when voltage from the primary DCsource 10 (e.g. a fuel cell bank) is at its lowest value, in this case50 VDC. For the sake of readability, only seven pulses of frequencymodulator 20 output are represented, although there are about 700 duringone 60 hz half period.

As shown in FIG. 2, at the output of the hyperbolic frequency modulator20, the waveform has a narrow rectangular shape. Then the reference halfsinewave (60 hz) are shown as formed after the precision rectifier 24.Then follows the sinusoidal output of the pulsewidth modulator 21 andthereunder are shown the output waveforms before the synchronousswitching 17 with voltage at point “C” being 162.01 VPK and at point “D”being 161.61 VPK. Finally, the waveform at load 18 after the synchronousswitching 17 is shown at the bottom of FIG. 2, producing a puresinusoidal waveform of constant amplitude (115 VAC rms) and a constantfrequency (60 hz).

FIG. 3 shows the main waveforms when the primary DC source 10 deliversits maximum DC voltage, in this case 100 VDC. For the sake ofreadability, only 4 pulses of the output of the frequency modulator 20are represented, but there are about 350 during one 60 hz half period.

As shown in FIG. 3, at the output of the hyperbolic frequency modulator20, the waveform has a narrow rectangular shape. It is similar to thewaveform shown in FIG. 2, but there are only 4 pulses for the periodwhere 7 pulses were produced at the minimum DC voltage. The referencehalf sinewave (60 hz) after the precision rectifier 24 are shown underthe hyperbolic frequency modulator output. Then follows the output ofthe sinusoidal pulsewidth modulator 21 and thereunder are shown theoutput waveforms before the synchronous switching 17, with voltage atpoint “C” being 62.61 VPK and at point “D” being 161.61 VPK which isexactly the same as in FIG. 2 for the minimum DC voltage. Finally, thewaveform at load 18 after the synchronous switching 17 is shown at thebottom of FIG. 3, producing as in FIG. 2, a pure sinewave, 60 hz, 115VAC rms, line and load regulated.

Obviously, for all intermediate values between minimum and maximumvoltages from the primary DC source 10, the output of the inverter willalso be a pure sinewave, 60 hz, 115 vac rms, line and load regulated. Itshould be noted that in this example, the primary DC source voltagevaries by a factor of 2 (50 VDC to 100 VDC). Hence, the hyperbolicmodulation curve fit has to be exact only over a 1 to 2 range. However,if the primary DC voltage were to vary by a factor of 5 (e.g. 20 VDC to100 VDC), the hyperbolic modulation fit would be exact over a 1 to 5range. This has been confirmed by calculations according to the formulaegiven in U.S. Pat. No. 5,357,418 as well as by numerous designsperformed by the applicant.

The invention is not limited to the specific embodiment and examplesdescribed above, but various modifications obvious to those skilled inthe art can be made without departing from the invention and thefollowing claims.

1. A sinewave inverter characterized in that it comprises a combinationof an open loop hyperbolic frequency modulator with a sinusoidalpulsewidth modulator followed by a linear regulator, producing a lineand load regulated distortionless sinusoidal voltage.
 2. A sinewaveinverter according to claim 1, in which the hyperbolic frequencymodulator is adapted to produce high frequency which is exactlyinversely proportional to a variable input DC voltage.
 3. A sinewaveinverter according to claim 1, in which the sinusoidal pulsewidthmodulator is adapted to produce a pulsewidth exactly proportional to thevoltage of a sinusoidal distortionless reference voltage from a puresinewave modulator.
 4. A sinewave inverter according to claim 1, inwhich the sinusoidal pulsewidth modulator is adapted to produce avoltage which is exactly proportional to the voltage from a grid,thereby enabling the inverter to produce AC voltage which exactly mimicsthe grid voltage amplitude, frequency and waveshape and hence candeliver power to the grid.
 5. A sinewave inverter according to claim 1,further comprising a precision full wave rectifier adapted to provide areference signal from a master-slave arrangement suitable to deliver anydesired power output.
 6. A sinewave inverter using a hybrid regulatorfor converting DC input voltage from a variable DC source to puresinewave line and load regulated AC voltage at the output, whichcomprises: (a) a hyperbolic frequency modulator for producing highfrequency which is exactly inversely proportional to the variable inputDC voltage; (b) a voltage divider for feeding a fraction of the inputvoltage to said hyperbolic frequency modulator; (c) a sinusoidalpulsewidth modulator producing a pulse triggered by the modulatedfrequency from the hyperbolic frequency modulator and whose width isexactly proportional to the reference half sinewave amplitude from aninternal or external sine reference source and a precision full waverectifier; (d) a pair of push-pull switching FETs connected to abi-phase toggle which is triggered by the sinusoidal pulsewidthmodulator and the hyperbolic frequency modulator and providing aflip-flop for the two phases of FET drives of the push-pull stage; (e) ahigh frequency transformer following the push-pull stage connected to anintegrating choke which itself is connected to a FET pass element usedto produce a low drop linear regulator which is provided with anamplifier whose reference input receives half-sine waves from the linearregulator; and (f) a FET synchronous bridge for converting the amplifiedhalf sine waves obtained from the linear regulator into full sinewavesof AC voltage at the output of the inverter.
 7. A sinewave inverteraccording to claim 6, in which the input voltage is unstable DC voltageprovided by batteries, fuel cells, wind mills, photovoltaic cells orsolar cells.
 8. A sinewave inverter according to claim 6, in which thepure sinewave produced at the output has less than 2% harmonicdistortion.
 9. A sinewave inverter according to claim 6, in which thehyperbolic frequency modulator which produces a pulsewidth exactlyinversely proportional to the variable input DC voltage has anexactitude of ±1%.
 10. A sinewave inverter according to claim 6, inwhich the sinusoidal pulsewidth modulator which produces a pulse whosewidth is exactly proportional to the reference half sinewave amplitudefrom a reference source has an exactitude of ±1%.
 11. A sinewaveinverter according to claim 2, in which the sinusoidal pulsewidthmodulator is adapted to produce a pulsewidth exactly proportional to thevoltage of a sinusoidal distortionless reference voltage from a puresinewave modulator.
 12. A sinewave inverter according to claim 2, inwhich the sinusoidal pulsewidth modulator is adapted to produce avoltage which is exactly proportional to the voltage from a grid,thereby enabling the inverter to produce AC voltage which exactly mimicsthe grid voltage amplitude, frequency and waveshape and hence candeliver power to the grid.
 13. A sinewave inverter according to claim 2,further comprising a precision full wave rectifier adapted to provide areference signal from a master-slave arrangement suitable to deliver anydesired power output.
 14. A sinewave inverter according to claim 3,further comprising a precision full wave rectifier adapted to provide areference signal from a master-slave arrangement suitable to deliver anydesired power output.
 15. A sinewave inverter according to claim 4,further comprising a precision full wave rectifier adapted to provide areference signal from a master-slave arrangement suitable to deliver anydesired power output.
 16. A sinewave inverter according to claim 7, inwhich the pure sinewave produced at the output has less than 2% harmonicdistortion.
 17. A sinewave inverter according to claim 7, in which thehyperbolic frequency modulator which produces a pulsewidth exactlyinversely proportional to the variable input DC voltage has anexactitude of ±1%.
 18. A sinewave inverter according to claim 8, inwhich the hyperbolic frequency modulator which produces a pulsewidthexactly inversely proportional to the variable input DC voltage has anexactitude of ±1%.
 19. A sinewave inverter according to claim 7, inwhich the sinusoidal pulsewidth modulator which produces a pulse whosewidth is exactly proportional to the reference half sinewave amplitudefrom a reference source has an exactitude of ±1%.
 20. A sinewaveinverter according to claim 8, in which the sinusoidal pulsewidthmodulator which produces a pulse whose width is exactly proportional tothe reference half sinewave amplitude from a reference source has anexactitude of ±1%.